Update xd ui test task

 Rename ui test task to 'ui_test' so that gradle test won't
  call spring-xd-ui tests. To run the E2E tests, the ui_test
  task needs to be invoked explicitly.
 The 'ui_test' task will also build the UI and runs the unit tests
  as it depends on 'grunt_build'

 Once we have valid UI tests, we will rename the task ui_test to 'test'
  so that will run as a 'test' task.